Radiotherapy uses ionizing radiation to treat or destroy cancerous tumours and lesions. The damage to tumour cells from the radiation is related to the absorbed dose (i.e., energy absorbed from ionizing radiation per unit mass). Therefore increasing the dose to the tumour increases the number of treated or destroyed cancer cells. However, as higher dose levels may also affect healthy tissue and other structures surrounding the tumour, the amount of ionizing radiation used must be controlled to provide as high as possible a dose to the tumour site whilst minimizing damage to the surrounding healthy tissue.
In three-dimensional conformal radiotherapy (3-D-CRT), a high dose of ionizing radiation is delivered to a tumour volume encompassing the tumour whilst delivering as low as possible a dose outside of this volume. This technique relies on accurately determining the location, size and shape of a tumour, planning the required radiation dose and delivery in order to effectively treat the tumour whilst minimizing complications related to the surrounding healthy tissue.
During the irradiation of intra-thoracic or near thoracic tumours or lesions, respiration can have a significant impact on the tumour location, shape and size and can therefore be a major contributor to the uncertainty in dose delivered to these tumours. It is understood that respiration can affect target and organ movements by as much as 12 mm in a cranio-caudal direction, 5 mm in a medio-lateral direction and 5 mm in a dorso-ventral direction.
Imaging techniques such as computed tomography (CT), magnetic resonance imaging (MRI) scans and other imaging methods have been used to attempt to locate tumours for the planning and execution of radiotherapy treatment, as well as for the calibration of the radiation delivery system prior to delivering a prescribed radiation to a patient. Typically, the location of a tumour is defined relative to implanted or anatomical markers. In CT scanning, for example, the three-dimensional anatomical description of the patient is acquired during free breathing and is used for planning the radiotherapy. However, this technique is affected by motion artefacts on the resultant images which makes localization of the tumour, and therefore the estimation of the dose to be received by the patient, prone to errors and inaccuracies.
Phantoms, defined as structures that contain one or more tissue substitutes used to simulate interactions and/or image properties of organs in the human body, are critical in the testing of the performance of imaging equipment, for measuring radiation dose during therapy and adjusting the real treatment accordingly, for interventional image guided procedures and for quality assurance testing.
One such lung phantom is described in U.S. Pat. No. 5,719,916 which is made of a spongy foam material having the desired x-ray opacity to simulate a human lung. It is placed in a torso cavity of a chest phantom on which breast phantoms can be adjustably attached. These phantoms are for calibrating mammography and x-ray equipment and as the torso cavity and the chest phantom are stationary, motion effects due to a patient breathing are not taken into account.
The recent emergence of four-dimensional radiotherapy, which is defined as “the explicit inclusion of the temporal changes of anatomy during the imaging, planning and delivery of radiotherapy”, necessitates phantom apparatus which can track the temporal changes of the anatomy during radiotherapy.
One such phantom apparatus is described in WO 2007/064951, which describes a human-like skeletal structure, deformable organ phantoms in the skeletal structure and a respiration actuator which is positioned to deform the deformable lung phantom with a respiration-like motion. The respiration phantom can be used to determine the amount of radiation exposure to a volume of interest during simulated breathing. In one embodiment, the respiration actuator includes a motor coupled to a push rod and a push plate to reciprocally compress the deformable organ phantoms in the skeletal structure along an inferior to superior axis. This causes the organ phantoms to simultaneously expand or bulge along a posterior to anterior axis and to press against a rib-cage of the human-like skeletal structure. However, this mechanism may not impart a physiologically correct movement of the organs during breathing.
Therefore, there is a need for an improved phantom apparatus which can simulate temporal anatomical changes.